Current and future environmental regulations necessitate design and operation changes to existing and new fired heaters and boilers used in many industrial processes, including the refining of petroleum. Conventional fired heaters, as traditionally operated, produce flue gases comprising water vapor (H2O), carbon dioxide (CO2), unburned fuel, nitrogen oxides (NOx), sulfur oxides (SOx) and nitrogen (N2). Of these exhaust gases, carbon monoxide (CO), unburned fuel, nitrogen oxides (NOx), and sulfur oxides (SOx) are objectionable environmental pollutants and/or health hazards.
More particularly, CO is an odorless toxic gas that causes a variety of physical ailments, including headaches, nausea, unconsciousness and, ultimately, death, upon prolonged exposure. NOx, which comprises NO, N2O3 and NO3, reacts with hydrocarbons in the presence of oxygen and sunlight to form a photochemical smog contributing to the “Green House” effect in the Earth's atmosphere. Unburned hydrocarbons also contribute to smog and the “Green House” effect in the Earth's atmosphere. SOx, which comprises SO2 or SO3, produces acid rain and is toxic.
In light of these harmful pollutants and environmental regulations, control of the pollutants has become a primary design parameter for fired heaters and other fuel combustion devices. However, heater design has not evolved to the level that satisfies environmental and health concerns while also satisfying the economic and technical practicalities of operating fired heaters.
Of particular and recent concern in heater design is the reduction of NOx. NOx reduction may be accomplished through the elimination of N2 as an oxidant during combustion processes. However, N2 is the major component of air, which provides the oxygen necessary for the combustion of hydrocarbons. Consequently, some of these efforts have suggested using pure oxygen in lieu of air to remove N2 from the combustion process.
Although using pure oxygen greatly reduces the presence of N2 during the combustion process, the use of pure oxygen presents additional problems not associated with the use of air to combust fuel. Using pure oxygen in a conventional fired heater results in elevated radiant section temperatures due to the absence of N2, which removes heat from the radiant section of fired heaters. Such elevated temperatures pose substantial safety and environmental risks including, but not limited to, heater material failure. Additionally, elevated temperatures may cause degradation of hydrocarbon process fluids that are heated in the tubes of the radiant section of the heater resulting in coking and unit downtime for many chemical and refining processes. If the fuel contains nitrogen or nitrogen enters the heater from the ambient air, these elevated temperatures also pose the environmental hazard of converting excessive quantities of the nitrogen to NOx. Additionally, the presence of the nitrogen may cause a reduction of fired heater efficiency, therefore requiring increased fuel consumption and increased CO2 emissions to the atmosphere.
Consequently, to moderate the radiant section temperature, some publications have suggested recycling combustion exhaust gases to the combustion process, alleviating the need for N2 as a temperature moderator. Wilkinson et al., CO2 Capture via Oxyfuel Firing: Optimisation of a Retrofit Design Concept for a Refinery Power Station Boiler, First National Conf. on Carbon Sequestration (May 2001), discloses a method for capturing CO2 from boilers for heating water by combusting fuel with pure oxygen and a flue gas recycle stream. Although the Wilkinson disclosure advances the art, the Wilkinson boiler is limited to boiler design and does not address critical design and operational aspects of heating hydrocarbon process fluids while reducing NOx emissions.
Additionally, the Wilkinson publication and similar efforts have not been widely commercialized, and attempts to design and commercialize such, efforts have revealed potentially catastrophic problems and challenges. For example, high temperatures due to rapid oxidation of fuels can be catastrophic in the absence of large quantities of nitrogen to remove heat from the radiant section of conventional fired heaters. In flue gas recycle designs, changes in fuel heat content or flue gas heat capacity can create dangerous conditions that are not realized until they are fully involved, which may jeopardize safe and economical operation of the fired heater.
We have now found that combusting fuel in a fired heater in the presence of pure oxygen within a combustion regime represented by
            4      ⁢              ,            ⁢      500        ≥          7554.8      -              933.72        ⁢        x            +              64.960        ⁢                  x          2                    +              .47705        ⁢        y            -              .55680        ⁢        z            -              1579.2        w              ≥          2      ⁢              ,            ⁢      500        ,wherein w is the mole percent of oxygen in the oxidizer, x is the recycle ratio of the flue gas as measured in moles of recycle flue gas/mole of oxidizer, y is the temperature (° F.) of the flue gas injected into the radiant section of the fired heater, and z is the heating value of fuel in the combustion process, as measured in Btu/scf LHV, results in unprecedented safe and environmentally sound operation of a fired heater.
We have also found that combusting fuel in the presence of substantially pure oxygen while recycling flue gases in a fired heater such that the radiant section of the heater operates at a pressure greater than ambient pressure results in substantially zero NOx produced from ambient air infiltration while maintaining safe and efficient operation.
We have also found that continuous online monitoring of the fuel composition, flue gas temperature and flue gas recycle rate allows for rapid and accurate control of fuel combustion in a fired heater within the combustion regime represented by
            4      ⁢              ,            ⁢      500        ≥          7554.8      -              933.72        ⁢        x            +              64.960        ⁢                  x          2                    +              .47705        ⁢        y            -              .55680        ⁢        z            -              1579.2        w              ≥          2      ⁢              ,            ⁢      500        ,resulting in unprecedented safe and environmentally desirable operation of a fired heater.